Urban dictionary is a dictionary that can be edited by anyone. Just a few days ago I continuously pressed the 'random word' button to see what it would come up with and found that almost every entry it gave was a joke entry.
If you come across regional slang you've never seen before, Urban Dictionary can be helpful in finding out what the slang means, but I certainly wouldn't rely on it. Always use it with caution, and only when no definition in a 'proper' dictionary fits t.
The first definition of 'rotten' that Urban Dictionary provides is the proper one:
"A word which can be used to describe or define something which is disgusting, yucky, smelly, unsightly or dirty."
It can also be used to describe a person with a bad personality.
I've never heard the other definition of rotten before. If it's ever used like that, it's definitely non-standard and very likely regional.